Porous PTFE membranes are generally produced as follows. A mixture obtained by mixing a PTFE fine powder and a liquid lubricant serving as an extrusion aid is extrusion-molded, and the resulting molded body is rolled into a sheet. The liquid lubricant is removed from the PTFE sheet thus obtained by rolling, and then the resulting PTFE sheet, from which the liquid lubricant has been removed, is stretched to make the sheet porous. The porous PTFE membrane thus obtained has a porous structure of nodes and fibrils, as is well known.
Housings of some electronic devices and lighting devices are provided with openings. In such an electronic device, acoustic energy propagates, through the opening, between an acoustic transducer such as a microphone or a speaker mounted in the housing and the outside of the housing. In the case of the housing of a lighting device, air that is expanded by heat produced from a light emitter is discharged to the outside of the housing through the opening. Since small-sized electronic devices such as mobile phones and vehicle lighting devices such as automotive headlights require high waterproofness in some cases, water intrusion through the openings needs to be prevented. Therefore, waterproof air-permeable members having both water resistance and air permeability (sound transmittance) are often disposed at the openings of the housings of these devices.
A waterproof air-permeable member used in an electronic device is also referred to as a waterproof sound-transmitting member, but hereinafter in this description, the term “waterproof air-permeable member” is used as a term representing the concept including a waterproof sound-transmitting member.
The performance of a porous PTFE membrane for use as a waterproof air-permeable member is evaluated using the water resistance and air permeability as indicators, but these two properties are in a so-called trade-off relationship. Therefore, there have been proposals to use a multilayer porous PTFE membrane so as to provide a waterproof air-permeable member having both excellent water resistance and excellent air permeability.
Patent Literature 1 proposes that a porous PTFE membrane be produced by applying a pressure to a laminated body of a first unsintered sheet made of PTFE having a standard specific gravity of 2.16 or more and a second unsintered sheet made of PTFE having a standard specific gravity of less than 2.16 to pressure-bond them, and further stretching the pressure-bonded article. Porous PTFE membranes having excellent air permeability tend to be obtained from PTFE having a high standard specific gravity, in other words, having a low molecular weight. Porous PTFE membranes having excellent water resistance tend to be obtained from PTFE having a low standard specific gravity, in other words, having a high molecular weight. In view of this tendency, in Patent Literature 1, the above-mentioned two types of PTFE sheets are used in combination to achieve a good balance between water resistance and air permeability. Patent Literature 1 reports that porous PTFE membranes each having a water entry pressure of 0.31 to 0.33 MPa and an air permeability of 3 to 5 sec/100 ml in terms of Gurley number (equivalent to about 0.31 to 0.52 cm3/sec/cm2 in terms of Frazier number) were obtained in Examples.